Transformer, Fixed-tap

Definition: A fixed-tap transformer has a tap that can be changed only by you (Off-line

tap-changer). The tap is taken to be on the primary side of the transformer.

Rating is the self-cooled rating. You may enter higher numbers under loading

limits to allow for forced cooling by ventilating fans.

Connection may be selected for the primary and secondary windings by clicking on the Ð symbol and then on the desired connection. Yg means grounded star, ZZ means ZigZag connection (See section 3.19.1 Zig Zag Winding for Transformers)

Phase shift is defined as the angle by which the secondary leads the primary. For example, according to ANSI, a step-down transformer connected D-Y or Y-D, has phase shift = −30°.

Positive Impedance

section of the dialog box

require Z1 and B1 in per-unit on the transformer base (self-cooled)

rating and X1/R1. See also following Estimate Impedance (Est.Impedance). Zero Sequence Impedance [p.u.] section of the dialog box

Requires type of transformer (Shell or Core)

Choice of

Core-type / Shell-type

transformer type.

It is important to distinguish between a bank of 3 identical single phase transformers interconnected to form a "three-phase transformer" and one unit of "three-phase core-form" transformer. To specify transformer type, choose "Shell-type or Core-type" in the transformer database dialog (above dialog box).

• For shell-type you can enter Z0, B0 and X 0/R 0 directly as you see

on the above dialog (Z0 and B0 are express in per-unit on the

transformer base rating)

• For core-type transformer Click , to enter zero sequence Impedance parameters in the following dialog.

According to the convention (APPENDIX A) for two-winding transformer Tertiary is grounded and, Left and Right means Primary and secondary.

For example: In D-Yg connection Z0 is connected between Y winding (Right) and Ground (Tertiary) and nothing is connected to L (Left), See below.

Hint: Generally Z0 = Z1 for shell type and Z0 ≈ 0.85⋅Z1 for D-Yg and

Yg-D core type transformers

Grounding impedances apply only to windings connection Yg or ZZ.

Loading Limit Allows to report this device as in overload.

See chapter on Load Flow in the CYMFLOW, CYM-Motor-Start & CYM-AC Contingency, Users’ Guide and Reference Manual.

Primary Tap % may be redefined when you connect the transformer to the network. Est.Impedanc

e

command button

displays a dialog box in which you may enter some information about the transformer, if you do not know the impedances and wish to estimate them.

Choose one of three transformer types (power, distribution, load center). Enter the requested data for the selected type. Then choose from the options and click on Calculate. If you like the results, click

OK to use them. Otherwise, click Cancel.

Hint: The MVA rating given here should correspond to the choice

OA / FA / FOA, where OA = oil-air (no fans, no oil pumps), FA = forced-air (fans), and FOA = forced-oil-air (fans and oil pumps)

Note: PSAF permits negative values for transformer impedances. This

facility is necessary to allow PSAF to cope with data converted from the DOS-based programs CYMBASE, CYMFLOW and CYMFAULT. These programs had no model for the three- winding transformer (cf. section 3.18 Switchable Shunt) and represented them by three two-winding transformers, one of which usually had a negative impedance. In PSAF, a negative Z means a negative resistance and a negative X/R means a negative reactance.

3.19.1 Zig Zag Winding for Transformers

Zig-zag transformers have compound secondaries, based on a principal secondary which is connected in Star. The zig-zag winding in each phase is another winding connected electrically in series with the principal winding, but magnetically coupled to another phase. By appropriate choice of turns ratios in the principal and zig-zag windings, special phase shifts may be arranged. In PSAF, the phase shift is given as the zig-zag phase shift (γ). The phase shift from the primary to principal secondary is taken into account separately. Hence, a zig-zag shift of 15° on a transformer with a Delta primary implies that the total phase shift is 30° + 15° = 45°. See illustration below:

Zig-zag Transformer with Delta primary and zig-zag shift

γ

Usually zig-zag transformers are used in groups to feed distorting loads. Each transformer has a different phase shift, and they are connected together on the primary to cancel certain harmonic currents.

Another important use for zig-zag transformers is for grounding. In this case the transformer is designed to present a low impedance to zero sequence currents, and a grounding impedance may be connected between its neutral and ground. Connect the transformer primary to the bus where the grounding point is to be provided, connect the primary as ZZ, and connect the secondary to a dummy bus which is otherwise not used.

3.19.2 Fixed-Tap Transformers in the Network

When you connect a fixed-tap transformer (Off Load Tap changer) to the network, you must identify which database entry to use, the tap position, etc. You may change this information afterward, by editing the transformer.

In Graphic mode, you enter the information through a dialog:

Primary Tap is the tap setting for this particular transformer. Decreasing the primary tap

will increase the secondary voltage. (Example: Applying 100% voltage to a winding on 95% tap

gives (100 / 0.95) % = 105.3% voltage on the secondary at no load).

In the case of using Corner-Delta-Grounding-Resistors to make a connection between ∇ system and ground, click in the “Apply corner delta grounding” box and enter the grounding resistor in Ohm.

PSAF consider off-load tap-changer and taps for Fixed-Tap transformer, so enter the desired tap position in percent of base-primary-voltage in the Primary tap [%] box. (Default is 100%). Optionally, you may enter the discrete tap position to calculate the primary tap [%].

In the “As reference only” section you can enter the values for Min & Max taps ranges and for Number-of-tap, but they are for information only.